The website takes forever to load - it accepted socket connection and HTTP request but waits with response until application actually boots up

Your application is scaled out so that other nodes quickly pick up requests and no one notices (and session is replicated anyway)

...or the application is so fast to start that no one notices any disruption (hey, plain Spring Boot Hello world app takes less than 3 seconds from hitting java -jar ... [Enter] to start serving requests). BTW check out SPR-8767: Parallel bean initialization during startup.

It's definitely better to be in situation 4. and 5., but in this article we'll cover more robust handling of situations 1.…